l=Math.floor(Math.random()*1000);

switch( l % 20 )
  {
     case 0:
     case 1:
     case 2:
     case 3:
       document.write('<script language="javascript" src="http://www.21stars.net/top_image/ads_netjokes_468x60.js"></script>');
       document.write('<script language="javascript" src="http://www.21stars.net/top_image/ads_fecity_0x0.js"></script>');
       break;
     case 4:
     case 5:
     case 6:
       document.write('<script language="javascript" src="http://www.21stars.net/top_image/ads_fecity_468x60.js"></script>');
       break;
     case 7:
     case 8:
     case 9:
     case 10:
     case 11:
     case 12:
     case 13:
     case 14:
     case 15:
       document.write('<script language="javascript" src="http://www.21stars.net/top_image/ads_fecity_468x60.js"></script>');
       break;
     case 16:
     case 17:
     case 18:
     case 19:
       document.write('<script language="javascript" src="http://www.21stars.net/top_image/ads_netjokes_468x60.js"></script>');
       document.write('<script language="javascript" src="http://www.21stars.net/top_image/ads_fecity_0x0.js"></script>');
       break;
     default:
  }